1. D. B. Dhaigude, Comparison theorems for a class of elliptic equations of order 4 m, Chinese J. Math., 15(1987), 61-67.
2. D. B. Dhaigude and D. Y. Kasture, Comparison theorems for nonlinear elliptic systems of second order with applications, J. Math. Anal. Appl., 112(1985), 178-189.
3. D. B. Dhaigude and M. R. Gosavi, Maximum principles for fourth order semilinear elliptic equations and applications, Differ. Equ. Dyn. Syst., 12:3(4)(2004), 279-287.
4. D. R. Dunninger, Maximum principles for solutions of some fourth- order elliptic equations, J. Math.Anal. Appl., 37(1972), 655-658.
5. C. Miranda, Formule di maggiorazione e teorema di esistenza per le funzioni biarmoniche di due variabli, Giorn. Mat. Battaglini, 78(1948), 97-118.